A load circuit that supplies electric power to a load such as a bulb and a motor, which is mounted on a vehicle, includes a battery and an electronic switch (MOSFET and the like) provided between the battery and the load. Then, the battery, the electronic switch and the load are connected to one another through conductors including electric wires. Moreover, a control circuit for performing ON/OFF operations for the electronic switch is provided, and drive and stop signals outputted from the control circuit perform the ON/OFF operations for the electronic switch, and switch the drive and stop of the load.
In the load circuit as described above, a fuse is provided for protecting the load, the electric wires, the electronic switch and the like by breaking the circuit instantaneously when an overcurrent flows through the load (refer to Patent Citation 1).
In a conventional load circuit shown in FIG. 1, power supply-side terminals of loads 101 are connected to a battery VB through an automotive electronic control unit (ECU) 102 and a junction box (J/B) 103.
Then, a plurality of electronic switches Tr1 such as the MOSFETs are provided in the ECU 102. These electronic switches Tr1 are controlled to be ON/OFF by a control IC 104. Moreover, first fuses F1 are provided on an upstream side of the respective electronic switches Tr1. These first fuses F1 protect electric wires W101 on a downstream side thereof. In other words, the electric wires W101 provided on the downstream side of the first fuses F1 have an electric wire diameter (cross-sectional area) sufficient for enduring a breaking current of the first fuses F1.
Moreover, second fuses F2 are provided in the J/B 103. These second fuses F2 protect an electric wire W102 on a downstream side thereof. In other words, the electric wire W102 provided on the downstream side of the second fuses F2 has a diameter (cross-sectional area) sufficient for enduring a breaking current of the second fuses F2.
Here, for example, in the case where the bulbs are used as the loads 101, the fuses F1 and F2 are deteriorated by rush currents generated when the bulbs are turned ON and by repetition of ON/OFF of the bulbs. Then, in some cases, erroneous breakdown occurs in the fuses F1 and F2 owing to a deterioration of the fuses F1 and F2, which is caused by use thereof with time. In order to prevent an occurrence of such a trouble, fuses prepared considering a margin for a load current are selected. Specifically, fuses in which the breakdown currents are increased somewhat more than usual are used. As a result, it is necessary to use electric wires adaptable to characteristics of the fuses prepared considering the margin, and it has become difficult to reduce the diameter of the electric wires for use in the load circuit.    Patent Citation 1: US 2003/0202304 A1